1) Field of the Invention
The present invention generally relates to the method of manufacture of metal bellows of the type used to form a fluid reservoir in implantable infusion pumps, and more particularly, to the method of manufacture of such metal bellows having an intermediate plate to aid in accurate sensing of the volume of the bellows.
2) Discussion of Related Art:
Extremely thin-gauge titanium welded bellows are commonly used in implantable infusion pumping devices as the main drug reservoir. A pump of this type is implanted within a patient and is used to accurately administer a precise dose of a stored liquid medication either continuously or at carefully timed intervals. As is well known in the art, within these small pumps, a metal bellows holds a supply of a liquid medication and is located within a sealed rigid propellant chamber containing a pressurized gas. The surrounding pressure differential within the chamber applies an even force to the bellows, which in turn collapses against the liquid medication within in a controlled and predictable manner. The collapsing bellows applies an even force to the contained liquid medication and in doing so, forces the liquid through controlling conduits and valves located elsewhere within the pump, eventually expelling the liquid medication to a target site within the patient. The liquid typically passes through what is called a micro-channel restrictor so that even though the pressure exerted on the liquid medication is very high, it dispenses from the pump at an extremely slow and controlled rate.
Since these pumps are surgically implanted within a patient, they are for the most part, inaccessible for long periods of time. For this reason, electrical and magnetic signals are typically used as a means of communication between the different sensing and controlling systems within the pump and remote monitoring devices located outside the patient's body. One such sensing system is used to detect the amount of medicament remaining the fluid reservoir of the pump. For obvious health reasons, it is important to know the moment the medicament level falls to a predetermined threshold so that the patient is never without treatment, as prescribed. When the threshold is reached, a piercing conduit can be used to replenish the liquid medicament supply within the bellows by penetrating the patient's skin and an additional septum within the pump.
Although there are likely different sensing methods that could be used to determine the amount of medicament within the fluid reservoir of these implantable pumps, one preferred system is disclosed in U.S. Pat. No. 6,755,814, which is commonly owned by the assignee of the present invention.
U.S. Pat. No. 6,755,814 discloses an implantable infusion pump that has a reservoir level detector. The pump includes a housing having a base plate which separates the housing into a pump electronic chamber and a propellant chamber. A bellows mechanism is disposed within the propellant chamber. The bellows mechanism has a bottom plate and defines a collapsible fluid reservoir into which the medicament for delivery to a patient is stored. A propellant is disposed about the bellows mechanism within the propellant chamber. The propellant compresses the bellows mechanism and thereby pushes the medicament out of the bellows mechanism through a flow restrictor, a valve and an outlet of the pump. As the bellows empties, the bottom plate naturally advances towards the base plate of the pump.
According to U.S. Pat. No. 6,755,814, a sensing circuit including a capacitor and a coil disposed within the base plate is used to form a resonant circuit. When energized, the coil generates a primary electromagnetic field, which flows through the bottom plate of the bellows mechanism and induces eddy currents therein. The strength of these eddy currents increases as the bottom plate moves closer to the coil within the base place. The eddy currents generate a secondary magnetic field, which is coupled back to the primary field. The closer the bottom plate is to the coil, the stronger the secondary magnetic field is and its influence on the primary field. This flux coupling brings about change to the inductance of the coil and thus brings about a displacement or shift of the resonance frequency of the resonant circuit depending on the distance between the coil and the bottom plate. Upon measuring the resonance frequency, which is dependent upon the inductance, well known circuitry can be employed to calculate the distance that the bottom plate of the bellows mechanism is from the base plate. This distance can then be used to determine the effective volume of the fluid reservoir and also the amount of medicament remaining within the bellow mechanism. An appropriate circuit can use this information to selectively create an alarm signal in response to a predetermined resonant frequency being reached.
In the prior art, such as the apparatus disclosed in U.S. Pat. No. 6,755,814, the measurement of fluid remaining in the reservoir is only accurate for the last 20 ml of fluid within the bellows mechanism. At fluid levels greater than about 20 ml and because of the increasing distance between the coil and the bottom plate at those greater volumes, the measured inductance doesn't vary sufficiently to provide accurate measurements.
In an effort to solve this problem, Applicant has filed US Patent Application No 2007/0106280 on Oct. 24, 1006, based on a provisional case filed Oct. 31, 2005, which is also commonly owned by the assignee of the present invention. U.S. Patent Application No. 2007/0106280 discloses an implantable infusion pump having the same main parts and operating in basically the same manner as that described above in U.S. Pat. No. 6,755,814, but further including an “intermediate plate”. This intermediate plate is secured within the bellows structure and inside the fluid reservoir and works with the resonant sensing circuit so that the volume of fluid within the bellows mechanism can be measured with a greater degree of accuracy, not only above 20 ml, but also within the critical range of between 0 and 20 ml.
To help explain the present invention, a quick discussion of the basic structure and operation of the pump of US Patent Application No 2007/0106280 is in order. To this end, referring now to FIG. 1 of the present application, an implantable pump 10 in accordance with the invention disclosed in U.S. Patent Application No 2007/0106280 is shown. Pump 10 has a housing 12. The housing is comprised of a base plate 14 and a can 13. Can 13 is attached to the base plate 14. Base plate 14 divides the housing into an electronics chamber 16 and a propellant chamber 18. A bellows mechanism 20 is connected to the base plate 14 and is disposed within the propellant chamber 18. The bellows mechanism 20 has a hermetically sealed expandable sidewall 22 and a bottom plate 24. The bellows mechanism 20 divides the propellant chamber into a medicament-receiving portion 26 and a non-medicament receiving portion 28. In one preferred exemplary embodiment of U.S. Patent Application No. 2007/0106280 a propellant located within propellant-receiving portion 28 applies an even force the adjacent bellows 20 causing the medicament within medicament-receiving portion 26 to be delivered to an outlet of pump 10 in a manner known to those skilled in the art. Alternatively, the pump maybe an active pump, such as, for example, a peristaltic-type pump, and the medicament in medicament-receiving portion is in fluid communication with the peristaltic pump conduit so that medicament is drawn from medicament-receiving portion 26 and subsequently delivered to the target site by way of the conduit. In this embodiment of U.S. Patent Application No. 2007/0106280, portion 28 may contain no propellant at all or a relatively small amount of propellant.
Regardless, in either case, the bellows mechanism of U.S. Patent Application No. 2007/0106280 includes an intermediate plate 30 disposed within the medicament-receiving portion 26. Intermediate plate 30 has at least one through-hole 32 therein to permit medicament to pass there-through. In a preferred exemplary embodiment of U.S. Patent Application No. 2007/0106280, intermediate plate 30 has four symmetrical through-holes 32. Those skilled in the art will readily appreciate that numerous other configurations can be used for intermediate plate 30 so long as they have the functionality to practice the specific embodiment. For example, plate 30 could be in the form of a grid. Plate 30 is preferably made of a biocompatible, non-magnetic material, such as, for example, titanium. According to U.S. Patent Application No. 2007/0106280, plate 30 could also be made of a combination of materials, such as, for example, a sandwich or layers of different materials, with the outer layer being biocompatible. U.S. Patent Application No. 2007/0106280 also discloses that intermediate plate 30 is preferably positioned 25% to 50% of the distance from the base plate 14 to the bottom plate 24 of bellow mechanism 20 at free length (e.g., when the bellows is in a stable state during its manufacturing). More preferably, intermediate plate 30 is disposed 33% to 40% of the distance from the base plate 14 to the bottom plate 24 of bellow mechanism 20. In a currently preferred exemplary embodiment, intermediate plate 30 is disposed approximately 40% of the distance from the base plate 14 to the bottom plate 24 of bellow mechanism 20. Thus, referring now to FIG. 1, the distance b divided by distance a (i.e., ratio b/a) is preferably 0.40, or 40%.
U.S. Patent Application No. 2007/0106280 further discloses that a coil 34 is disposed in a recess 36 on the lower surface 38 of base plate 14. A mu-metal 40 is disposed between coil 34 and base plate 14. Mu metals are nickel and iron alloys (usually 75% nickel, 15% iron and include copper and molybdenum) that offer very high magnetic permeability. This mu-metal 40 acts as a rear shield of the coil to limit the eddy current in the base plate 14. In addition, coil 34 is spaced from the internal wall, which is preferably made of titanium, by a distance. Coil 34 is isolated from the medicament chamber with a biocompatible titanium ring 35.
In a currently preferred exemplary embodiment of U.S. Patent Application No. 2007/0106280, the pump housing 12 is made of titanium. In addition, as stated above, intermediate plate 30 is also preferably made of titanium. The sensitivity in detecting the intermediate plate 30 increases with increasing thickness of plate 30. However, increasing the thickness of plate 30, increases the weight of the device and decreases the internal volume of the reservoir in the pump because intermediate plate 30 is disposed within the bellows reservoir medicament-receiving portion 26. The plate may have a thickness ranging from 0.2 mm to 0.7 mm, with 0.5 mm being preferred in a currently preferred exemplary embodiment. The value of the inductance seen across coil 34 is affected by the location of the intermediate plate 30 and bottom plate 24. The resonant frequency of the circuitry in which the coil 34 is placed is influenced by the inductance across coil 34. The amount of fluid remaining in the reservoir is determined based upon the measurement of the resonant frequency, which is correlated to the inductance. Currently pending and commonly owned U.S. patent application Ser. No. 1/278,048, filed Mar. 30, 2006, and entitled “Methods and Devices for Monitoring Fluid of an Implantable Infusion Pump” discloses, inter alia, a manner of using a fluid level sensor to monitor the amount of fluid in a reservoir. The disclosure of pending U.S. application Ser. No. 11/278,048, as well as the disclosure of U.S. Patent Application No. 2007/0106280 are both in their entirety, hereby incorporated by reference.
It is a first object of the invention to provide a method for manufacturing the metal bellows fluid reservoir including the intermediate plate of the pump described in U.S. Patent Application No. 2007/0106280.